Some radiopharmaceuticals undergo decomposition due to either radiolysis or redox reactions, and hence exhibit undesirable instability. Non-radioactive kits for the preparation of radiopharmaceuticals, especially Tc-99m radiopharmaceuticals, may suffer from two types of instability:    (i) shelf-life instability of the non-radioactive composition over time,    (ii) instability of the radiopharmaceutical post-formation.In the case of Tc-99m, the latter is referred to as post-reconstitution instability. U.S. Pat. No. 4,451,451 discloses that para-aminobenzoic acid (pABA) and analogues are useful stabilisers for technetium non-radioactive kits, including kits for the preparation of 99mTc-complexes of diphosphonic acids.
Tc-99m-hexamethylpropyleneamine oxime (referred to hereinafter as 99mTc-HMPAO), is a radiopharmaceutical commercially available as a regional cerebral blood flow imaging agent. 99mTc-HMPAO is particularly unstable with respect to post-reconstitution stability. 99mTc-HMPAO is usually prepared from a lyophilised, non-radioactive kit which contains HMPAO and stannous ion. The function of the stannous ion is to reduce the 99mTc-pertechnetate (99mTcO4−), ie. technetium in oxidation state Tc(VII), to the Tc(V) oxidation state of the 99mTc-HMPAO metal complex. The radiochemical purity (Rcp) of 99mTc-HMPAO one hour after Tc-99m labelling is only about 80%, so that it must be used within 30 minutes of 99mTc labelling.
After Tc-99m radiolabelling, the Rcp of 99mTc-HMPAO decreases with time due to the growth of three different radioactive impurities, namely: a hydrophilic secondary 99mTc complex of unknown structure derived from 99mTc-HMPAO, 99mTc-pertechnetate (99mTcO4−) and reduced-hydrolysed-technetium [99mTc]. Of these impurities, both the secondary complex and 99mTc-pertechnetate are decomposition products of 99mTc-HMPAO; however, it is reported that the decomposition mechanisms are different (J. Nucl. Med. 29, 1568-1576, 1988).
The secondary complex is believed to be produced when the lipophilic 99mTc-HMPAO complex is exposed to excess unoxidised tin(II) (ie. stannous) remaining from the pertechnetate reduction step. On the other hand, the 99mTc-pertechnetate impurity is produced when 99mTc-HMPAO and the secondary complex are oxidised by the free radicals produced in solution by the action of radiation, ie. radiolysis of the solvent.
Accordingly, in order to inhibit the production of both the 99mTc-pertechnetate and secondary complex impurities, the addition of stabilisers has been disclosed. Thus, Nucl. Med. Biol. 7, 675-680 (1989); Eur. J. Nucl. Med. 16, 541 (1990); Eur. J. Nucl. Med. 20, 661-666 (1993) and Eur. J. Nucl. Med. 22, 1163-1172 (1995) all report attempts to stabilise 99mTc-HMPAO by the addition of either: gentisic acid, sodium decahydroxypyrophosphate, methylene blue, cobalt chloride or the like. In particular, the post-radiolabelling addition of methylene blue improves the Rcp of 99mTc-HMPAO to at least 80% at 4 hours post reconstitution. Similarly, the post-radiolabelling addition of cobalt chloride has been found to improve the Rcp of 99mTc-HMPAO at 6 hours post reconstitution to at least 80%.
The stabilisation mechanisms of 99mTc-HMPAO by methylene blue and cobalt chloride are believed to be essentially the same. Both are in redox equilibrium in solution, and oxidise excess tin(II), thus stabilising 99mTc-HMPAO. However, when the reducing tin(II) and methylene blue or cobalt chloride coexist in solution before the Tc-99m radiolabelling step, the tin(II) reductant is completely oxidised, so that the Tc-99m labelling becomes impossible because there is no longer any reducing agent present to reduce the Tc(VII) 99mTc-blue or cobalt chloride is used as a stabiliser for 99mTc-HMPAO, it must be added after the Tc-99m radiolabelling step and cannot be pre-mixed with the ligand (HMPAO) and 99mTc-pertechnetate. Accordingly, any kit for the preparation of 99mTc-HMPAO employing such stabilisers, must be composed of two vials (referred to hereinafter as a 2-vial kit). One vial is a freeze-dried vial containing the HMPAO ligand together with the tin(II) reductant and other excipients. The other is a vial containing the stabiliser (methylene blue or cobalt chloride). Thus, the most successful prior art methods of stabilising 99mTc-HMPAO to date all require the use of 2-vial kits.
When the kit for preparing 99mTc-HMPAO is a 2-vial kit, the radiolabelling operation is more complicated than for a single vial kit and comprises two steps:    (1) 99mTc-pertechnetate solution is added to the vial containing the HMPAO ligand and the resulting mixture is mixed by shaking;    (2) a stabiliser solution from the second vial (eg. methylene blue or cobalt chloride) is added to the reconstituted mixture from step (1) in the first vial.
It is necessary that the time between the first and second steps is controlled so as to be as close as possible to two minutes. Too short a time, and 99mTc-HMPAO complex formation may be incomplete and hence addition of the stabiliser may adversely affect the Rcp by oxidising the stannous ion before the reduction of the pertechnetate starting material is complete. Too long a time, and the stabilising effect is delayed. In such a procedure it is also necessary that care be taken with respect to the amounts of the solutions added. The operator must also take due care to ensure that the vials are not inadvertently mixed up at any stage. In addition, there is an increased risk of radiation dose to the operator due to the increased number of manipulations. Moreover, when methylene blue is added to 99mTc-HMPAO, a precipitate is produced, so that a filtration step becomes necessary, and thus the procedure becomes more complicated.
There is therefore a need for a single vial kit for the preparation of 99mTc-HMPAO which has both shelf-life and post-radiolabelling stability. The present invention provides a kit which solves this problem, and is straightforward to use.
The Present Invention.
The present invention relates to a stabiliser for radiopharmaceuticals which comprises a combination of an amino-substituted aromatic carboxylic acid or a salt, ester or amide thereof, with a diphosphonic acid or a salt thereof with the proviso that the radiopharmaceutical is not a metal complex of the diphosphonic acid.